In the past, nitrile rubber (acrylonitrile-butadiene copolymer rubber) has been used as a material for rubber parts for automobile such as hoses, belts, and tubes by making use of its oil resistance, mechanical properties, chemical resistance, etc. Further, highly saturated nitrile rubber obtained by saturation of carbon-carbon double bonds in the polymer main chain of nitrile rubber by hydrogenation etc. is further excellent in heat resistance and is being used for rubber parts such as seals, belts, hoses, and diaphragms.
In view of such a situation, Patent Document 1 discloses a method of improving the processability such as fluidity by causing a metathesis reaction of a nitrile group-containing copolymer rubber in the presence of a metathesis catalyst to thereby obtain a nitrile group-containing copolymer rubber lowered in molecular weight.
However, in the art of the above Patent Document 1, due to the reduction of the molecular weight, the obtained nitrile group-containing copolymer rubber is improved in fluidity, but when made into cross-linked rubber, the mechanical properties such as the normal physical properties are not sufficient. For this reason, not only excellent processability (fluidity), but also improvement of the mechanical properties of the obtained cross-linked rubber has been sought.